1. Field of the Invention
The present invention relates to a method and system for optical transmission adopting dispersion compensation.
2. Description of the Related Art
Owing to recent developments of low-loss silica optical fibers, various optical fiber communication systems each using such an optical fiber as a transmission line have been put to practical use. The optical fiber itself has a very wide band. However, a transmission capacity by the optical fiber is actually limited by a system design. The most important limitation is due to waveform distortion by chromatic dispersion occurring in the optical fiber. Further, the optical fiber attenuates an optical signal at a rate of about 0.2 dB/km, for example. Loss of the optical signal due to this attenuation has been compensated for by adopting an optical amplifier such as an erbium doped fiber amplifier (EDFA) that is a typical example. The EDFA has a gain band in a 1.55 μm band where a silica optical fiber gives a lowest loss.
The chromatic dispersion that is often referred to simply as dispersion is a phenomenon such that the group velocity of an optical signal in an optical fiber changes as a function of the wavelength (or frequency) of the optical signal. In a standard single-mode fiber, for example, an optical signal having a longer wavelength propagates faster than an optical signal having a shorter wavelength in a wavelength region shorter than 1.3 μm, and the resultant dispersion is usually referred to as normal dispersion. In this case, the dispersion (whose unit is ps/nm/km) takes on a negative value. In contrast, an optical signal having a shorter wavelength propagates faster than an optical signal having a longer wavelength in a wavelength region longer than 1.3 μm, and the resultant dispersion is usually referred to as anomalous dispersion. In this case, the dispersion takes on a positive value.
In recent years, the nonlinearities of an optical fiber have received attention in association with an increase in optical signal power due to the use of an EDFA. The most important nonlinearity that limits a transmission capacity is an optical Kerr effect occurring in an optical fiber. The optical Kerr effect is a phenomenon such that the refractive index of an optical fiber changes with the power or intensity of an optical signal. A change in the refractive index modulates the phase of an optical signal propagating in an optical fiber, resulting in the occurrence of frequency (wavelength) shift near the leading edge and the trailing edge of an optical waveform. This phenomenon is known as self-phase modulation (SPM). There is a possibility that such a change in spectrum due to SPM may further enlarge the waveform distortion due to chromatic dispersion.
In this manner, the chromatic dispersion and the optical Kerr effect impart waveform distortion to an optical signal with an increase in transmission distance. Accordingly, to allow long-haul transmission by an optical fiber while ensuring a transmission quality, the chromatic dispersion and the nonlinearity must be controlled, compensated, or suppressed.
As a method for compensating for dispersion in an optical fiber transmission system, a method using a dispersion compensator is known. For example, the dispersion compensator is configured so as to include a dispersion compensating fiber (DCF) having such a dispersion as canceling the dispersion of an optical fiber transmission line.
Known as a form of the optical fiber transmission system is a linear repeater system configured by connecting a plurality of segments each formed from an optical fiber and providing an optical amplifier at each connection point of these segments. This kind of system usually includes a plurality of dispersion compensators, and each dispersion compensator is provided in association with an optical transmitter, each optical amplifier, or an optical receiver. Each dispersion compensator is designed so that the total dispersion over the optical fiber transmission line is made to fall within tolerance by the addition of a dispersion given by each dispersion compensator. Accordingly, if there are variations in length of the optical fiber forming each segment, each dispersion compensator cannot be easily designed.
In another respect, the optical fiber to be used as the optical fiber transmission line is of various kinds such as a single-mode fiber (SMF) having a zero-dispersion wavelength of about 1.3 μm and a dispersion shifted fiber (DSF) having a zero-dispersion wavelength of about 1.55 μm. The SMF has a low loss in a wavelength band of 1.55 μm, but has a relatively large dispersion (e.g., 18 ps/nm/km) in this wavelength band. The SMF is widely installed at present, and can also support WDM (wavelength division multiplexing) transmission. On the other hand, the DSF is a fiber having a zero-dispersion wavelength shifted into a 1.55-μm band that is a low-loss wavelength band, and the amount of installation of the DSF is yet small at present. Further, the DSF is susceptible to nonlinearity in WDM transmission. To reduce the susceptibility to nonlinearity, an NZ (nonzero)-DSF having slight dispersion in the 1.55-μm band has also been developed.
Also in such a case that a plurality of segments formed from various kinds of optical fibers are mixed, the placement of dispersion compensators and the distribution of dispersion compensation amounts are important in system design, and each dispersion compensator cannot be easily designed.